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By Charles Rhodes, P.Eng., Ph.D.

There are 4.5 m high buoyant indicator tubes field attached to the mobile active fuel bundles. The vertical position of each active square fuel bundle is visually indicated by the 0.3 m to 1.5 m exposed height of the top of its indicator tube above the primary liquid sodium surface.

Indicator tubes are attached to the mobile active square fuel bundles after the mobile active square fuel bundles are installed and are removed before the mobile active square fuel bundles are relocated. The indicator tube attachment point is the fuel bundle lifting point. Once the indicator tubes are in place the sodium isolation floats can slipped between them. The indicator tubes should be thin wall for buoyancy to keep each indicator tube upright. The indicator tube diameter should be minimal to minimize obstruction of liquid sodium flow, but must be sufficient to allow accurate fuel bundle liquid sodium discharge temperature measurement. The difference between the indicator tube OD and ID provides a path for gamma radiation to reach the FNR monitoring system.

A FNR fuel bundle lifting points are achieved by replacing the (3 / 16) inch thick diagonal plates with (3 / 8) inch thick diagonal plates in the upper portion to the fuel bundle where there are fuel tube plenums. The two (3 / 8) inch thick plates extend above the tops of the fuel tubes. Two 3.0 inch diameter holes in each plate form the lifting point.

The lifting points for a fuel bundle are pairs of holes in 0.375 inch thick diagonal plates. The corner girders of the octagonal fuel bundles must project upwards above the fuel tubes to allow for bolting to other fixed fuel bundles at the top of the corner girders. The diagonal plates connecting each fuel bundle lifting point to the corresponding fuel bundle corner girders must also allow unobstructed primary liquid sodium flow and must not prevent individual fuel tube insertion or extraction.

The indicator tubes must have bottom hooks that attach to the lifting points of the mobile square fuel bundles.

An indicator tube isolates a mobile fuel bundle's hot liquid sodium discharge stream from the temperature of the surrounding liquid sodium. This isolation ensures that the temperature in the middle of the indicator tube reflects the temperature of the fuel bundle sodium discharge. The hollow walls of the indictor tube also provide positive buoyancy so that when 1.5 m of the indicator tube is projecting above the primary liquid sodium surface the indicator tube still maintains a firm upright position.

Note that the buoyancy of the hollow wall indicator tube is not sufficient to lift the net weight of a mobile square fuel bundle when the indictor tube is fully immersed in liquid sodium. However, the indicator tube must be buoyant in liquid sodium even when at its maximum height.
Indicator Tube: 5.563 inch OD X 0.258 inch wall X 4.5 m long_______
Mass = Pi X 5.563 inch X 0.258 inch X 4.5 m X (.0254 m / inch)^2 X 7.874 X 10^3 kg / m^3
= 103.074 kg_______________

(Consider use of thinner wall material)

Each indicator tube: shows the actual vertical position of its corresponding mobile square fuel bundle, allows measurement of the gamma flux emitted vertically by the mobile square fuel bundle and allows determination of that fuel bundle's steady state discharge temperature.

The thermal power output from a fuel bundle is proportional to the gamma flux propagating up its indicator tube.

The steady state square fuel bundle discharge temperature is indicated by the liquid sodium temperature inside the indicator tube.

The fuel tube bundle frame and shroud are fabricated from HT-9 steel (85% Fe, 12% Cr, 1% Mo, 0% C, 0% Ni).

The height allowances for the octagonal fuel bundle components from bottom to top are: legs (1.5 m), bottom grating (0.1 m), fuel tubes (6 m), lifting point (0.3 m), swelling allowance 0.1 m. Hence the fuel bundle shipping container and the air lock tube must be able to accommodate a fuel bundle with an overall length of 8.0 m.

The present design provides an ideal 0.25 inch clearance between a mobile fuel bundle and each of the adjacent fixed fuel bundles. With good dimensional tolerance control this clearance should be sufficient to allow reasonable core zone material swelling.

7) The fixed octagonal fuel bundle maximum outside face to outside face distance is:
23 X (5 / 8) inch = 14.375.0 inches.

9) The square fuel bundle maximum outside face to outside face distance is:
19 X (5 / 8) inch = 11.875 inches.

11) To prevent overall fuel bundle swelling in the core region in that region the diagonal reinforcing sheets are reduced in width and the fuel bundle shroud sheets contain vertical slots to allow shroud and diagonal sheet swelling in the core region without causing significant overall horizontal fuel bundle width swelling.

21) An important issue in earthquake protection is bolting the fixed fuel bundles together to form a rigid matrix. We do not want liquid sodium sloshing back and forth to change the fuel assembly geometry and hence its reactivity.

Note that the open steel lattice near the bottom of the primary liquid sodium pool will thermally expand with increasing surrounding liquid sodium temperature. During normal reactor operation the open steel lattice is likely to be about 150 degrees C cooler than the liquid sodium temperature at the top of the fuel bundle. Hence the differential horizontal width thermal expansion per fuel bundle is approximately:
20 ppm / deg C X 150 deg C X 13.125 inch = 0.039 inch
The fuel bundle leg sockets must provide sufficient play to accommodate this differential thermal expansion.

The mobile fuel bundle travel is limited at the bottom by the probe length (1.2 m) and the hydraulic cylinder end piece and piston thickness (0.3 m) and height of the steel lattice (1.5 m) and at the top by a hydraulic actuator vent hole.

A practical FNR consists of a variable thickness pancake shaped inner core completely surrounded by a > 1.45 m thick neutron absorption blanket. The fission chain reaction occurs primarily in the core zone where the core fuel rods of the fixed and mobile fuel bundles overlap. Excess neutrons originating in the core zone are absorbed by U-238 in the blanket. The core and blanket vertical thicknesses are set by fuel rod lengths and by the amount of mobile fuel bundle insertion into the fixed fuel bundle matrix.

A key issue in fuel bundle design is that with half of the mobile fuel bundles fully withdrawn from the matrix of fixed fuel bundles and the remainder in their normal operating position the reactor must be subcritical. This fuel bundle design constraint enables safe reactor assembly/disassembly and allows independent operation of the two fully independent FNR cold shutdown systems. In order to achieve reactor zone symmetry the fissile fuel concentration in the mobile active fuel bundles is higher than the fissile fuel concentration in the fixed active fuel bundles.

The extent of insertion of a mobile square fuel bundle into the fixed fuel bundle matrix is determined by the volume of liquid sodium inside the fuel bundle's hydraulic actuator. There is fluid pressure feedback which indicates the approximate mobile fuel bundle vertical position due to the changing buoyancy of the indicator tube. The hydraulic fluid feed tube is routed through the open steel lattice. This hydraulic tube must be sufficiently flexible to allow for +/- 1 m ______earthquake induced movement of the primary sodium pool with respect to its concrete enclosure.

Volume = 4 X 0.25 inch^2 X 20 inch = 20 inch^3 ????

Mass = 20.0 inch^3 X (0.0254 m / inch)^3 X 7.874 X 10^3 kg / m^3
= 7.742 kg ?????__________

This web page last updated December 30, 2020

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